1. Field of the Invention
The present invention relates to communication cables containing at least one optical fiber within a tube, a space between the optical fiber and the tube being at least partially filled by a filling material.
2. Background of the Disclosure
Three general types of protective coatings have been recognized in the optical cable art: 1) loose tube buffers, 2) tight buffers and 3) composite buffers. See Mahlke, G. and Gossing, P., Fiber Optic Cables, John Wiley and Sons Limited, New York, 1987, pp. 96-108. Other specialized constructions have also been developed. See, for example, Saito et al., U.S. Pat. No. 4,770,489, which discusses the use of a grooved spacer core to carry a group of fibers, wherein the grooves are filled with a soft jelly-like material, the spacer is surrounded with a layer of aramid fiber, and the aramid fiber is surrounded by a plastic jacket. Filling compounds and flooding compounds are used within cables to block the flow of water therein but they are not wholly interchangeable. Filling compounds are suitable for contact with fibers and must not include a base composition or additives that result in unacceptable levels of optical performance changes.
Flooding compounds, on the other hand, typically include ingredients that render them unsuitable for contact with optical fibers. Generally filling compounds will be located in a tube having at least one optical fiber, and flooding compounds will be located exteriorly of the tube.
In the loose tube construction, one or more fibers are surrounded by a protective tube, typically referred to as a core or buffer tube, which is spaced from the fiber(s) by a distance sufficient to allow the fiber(s) to move within the core or buffer tube in response to applied stresses. By placing the fiber loosely in a tube, the fiber is substantially decoupled from the rest of the cable. Typically, the space between the fiber(s) and the loose tube is filled with a filling material, e.g., a grease or a thixotropic gel, so as to inhibit or prevent water migration in the tube and/or to protect the fiber(s) from water absorption.
Greveling, U.S. Pat. No. 4,763,982, discloses a cable comprising: 1) an outer jacket and 2) an inner tube composed of strength members, such as aramid fibers, embedded in resin carrier, the inner tube being filled with a thixotropic water blocking medium. Peacock, U.S. Pat. No. 4,822,133, discloses the use of water blocking grease used as a filling material in a tube. Other filling materials have included petroleum jelly mixed with a polymer, usually polyethylene, as described by Biskeborn et al, U.S. Pat. Nos. 3,607,487 and 3,717,716. Other filling materials have included a low viscosity oil thickened by a styrene-isoprene-styrene copolymer, again with polyethylene added to impart consistency and reduce slump; see, for example, Dobbin et al., U.S. Pat. No. 3,879,575.
Sabia, U.S. Pat. No. 4,464,013, describes the use of a mixture of (1) from 85 to 94 percent by weight of ASTM Type 103, 104A, or 104B, or mixtures thereof, naphthenic or paraffinic oil having a minimum specific gravity of 0.855; a minimum SUS viscosity at 210xc2x0 F. of 40; a maximum pour point ASTMD 97 of less than xe2x88x925xc2x0 F.; and a maximum of 5 percent aromatic oils; and (2) from 6 to 15 percent by weight of a styrene-ethylene butylene-styrene block copolymer having a styrene-rubber ratio of from approximately 0.2 to 0.5. These two ingredients comprise substantially 100 percent of the filling material, but with up to 1 weight percent of the filling material being stabilizer.
Patel, U.S. Pat. No. 4,497,538, proposes the use of a mixture of a block copolymer, petroleum and an additive. The block copolymer is a styrene-ethylene butylene-styrene (SEBS) having a styrene to rubber ratio of 0.39 to 0.41, and a specific gravity of approximately 0.91. The petroleum is a mixture of microcrystalline waxes and oil that contains a small amount of an antioxidant additive. The additive is a low molecular weight polyethylene having a molecular weight range from 1,000 to 10,000 and a specific gravity of at least 0.90.
A major component of the foregoing filling compositions is a petroleum based filling composition, for example, an oil, grease, or wax that has been mixed with minor amounts of one or more organic and/or inorganic compounds. Such petroleum based filling compositions are relatively expensive, require special blending and are not suitable for use all cable components. For example, not all plastics commonly used for cable components are adequately resistant to the petroleum based composition.
Another disadvantage of petroleum based filling compositions is that they may not be suitable in all environments. For example, a base petroleum composition may not form a network of molecular bonds that sufficiently reduces flow under normal cable use environments. As a result, petroleum based compositions are often messy, particularly for those working with the cable in the field. Moreover, known filling compositions are susceptible to dripping at high temperatures, and undesirably high optical performance losses at lower operating temperatures.
Accordingly, there is a need in the art for filling materials that, for example, are not as messy as petroleum based compositions, avoid dripping at high temperatures, and otherwise minimize optical performance losses.
Accordingly, the present invention relates to a cable containing at least one optical fiber within a core or buffer tube, a space between the optical fiber and the tube, and a filling material at least partially filling the space.
It is an object of the invention to provide an alternative to the petroleum based filling materials conventionally used in the art by providing a filling material that comprises a three-dimensional molecular network thermoplastic polymer that have bonded through crystallization, intermolecular hydrogen bonding and/or intermolecular polar attraction of functional groups and not necessarily through covalent crosslinking. The three dimensional network of thermoplastic polymer, optionally including various additives, desirably possesses suitable properties under service conditions such that temperature change does not cause large compressive strain on the fibers, desirably possesses a low modulus thereby minimizing the resulting strain on the optical fiber, inhibits dripping at high temperatures and is inexpensive.
It is a further object of the invention to provide an alternative to the petroleum based filling materials which possesses other desirable characteristics. For example, the thermoplastic filling material desirably does not slump excessively at elevated service temperatures, so that the material will not migrate out of the end of the tube. Further, the thermoplastic filling material is desirably not excessively oily or greasy, is not messy, and desirably has a soft-rubber texture allowing for convenient handling during cable repair and other operations.
It is a further object of the invention to provide an alternative to the petroleum based filling materials which is easily manufactured and incorporated in a tube. The thermoplastic material desirably possesses a melt flow at temperatures over about 80xc2x0 C., preferably between about 90xc2x0 C. and about 125xc2x0 C., yet remains sufficiently soft at temperatures as low as about xe2x88x9250xc2x0 C. so as not to cause excessive optical attenuation. By use of the thermoplastic material, slicing of the tube may be employed for ease of cable manufacture without the ordinary dripping considerations.
It is a further object of the invention to provide filling material with a three-dimensional network that may include controlled amounts of any or substantial amounts of petroleum based oils, greases and waxes, and can exclude additives often employed with the oils, greases and waxes. Additives that may be excluded from the filling material of the invention include fumed silica and other silica products. However, superabsorbent materials such as hydrophilic polymers and other stabilizers often used with conventional petroleum based filling materials may be desirable for use with the present invention.
It is a further object of the invention to provide an alternative to the petroleum based filling materials which may include a foam. In this embodiment, the three dimensional network of thermoplastic polymer includes a foam incorporated therein by conventional means. These means include, for example, injection of gas into the thermoplastic polymer, adding materials to the thermoplastic polymer which split off a gas when heated, dissolving a gas in the thermoplastic material under pressure, adding microspheres, as well as other mechanical and chemical means.